Method of forming electrical



April18 ,1944.: LUNAS 2,347,058

METHOD OF FORMING ELECTRICAL MEASURING INSTRUMENTS Original Filed Oct. 13, 1939 2 Sheets-Sheet 1 WITNESSES! INVENTOR I w [amylase/lanai ATT RNEY April 18, 1944;

WITNESSES:

L. J. LUNAS ,347,058

METHOD OF FORMING ELECTRICAL MEASURING INSTRUMENTS 2 Sheets-Sheet 2 Original Filed Oct. 13. 1939 INVENTOR [aw/Wire} [0220.57

M ATTORNEY Patented Apr. 18, 1944:

METHOD OF FORMING ELECTRICAL MEASURING INSTRUMENTS Lawrence J. Lunas, Cedar Grove, N. J., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Original application October 13, 1939, Serial No.

299,284. Divided and this application Decem- I her 24, 1941, Serial No. 424,306

Claims.

This invention relates to measuring instruments and it has particular relation to permanentma net DArsonval instruments.

This application is a division of my copending application, Serial No. 299,284, filed October 13, 1939, which issued September 8, 1942, as Patent 2,295,413.

Many measuring instruments employ magnetic structures for providing paths for magnetic flux employed in the operation of the measuring instruments. A permanent magnet DArsonval instrument is a representative of this class of instruments. The conventional DArsonval instrument comprises a permanent magnet and a pole-piece assembly for establishing an intense magnetic field in an annular gap. A coil is mounted for rotation in this gap in accordance with current passing therethrough. Because of the intensity of the magnetic field and the resulting high sensitivity of the measuring instrument, such an instrument is extremely sensitive to slight variations in the registration of its component parts. Slight discrepancies in the location of parts, such as those occasioned by variations in the dimensions of parts during the manufacture of an instrument, may result in a lack of uniformity in the response of the instrument to quantities to be measured. Furthermore, such variations in dimensions are objectionable for the reason that parts produced for such instruments may not be interchangeable. A still further variation in the properties of a DArsonval instrument is caused by the variation in multi-part pole-piece assemblies produced by temperature changes, vibration or other causes.

In accordance with my invention, parts which must be registered. accurately are provided with coacting surfaces of revolution. In a DArsonval instrument, it is important that bearing brackets be positioned accurately on a pole-piece assembly. For this reason, I provide a bearing bracket and a pole-piece assembly with coacting male and female surfaces for registration purposes. Preferably, these surfaces are'surfaces of revolution concentric with the annular gap customarily employed in such instruments. The pole-piece assembly preferably is constructed from a unitary homogeneous block of magnet material in order to preclude variations in the air gap caused by movements of component parts.

In order to position a magnetic core accurately in an opening provided in a pole-piece assembly, I first machine a magnetic core accurately and subsequently attach temporarily a spacer thereto.

It is, therefore, an object of my invention to provide an improved method for constructing measuring instruments. I

It is another object of my invention to provide a method for positioning partsof measuring instruments by forming coacting surfaces of revolution thereon.

It is another object of my invention to provide an improved method for assembling the parts of a DArsonval instrument.

It is another object of my invention to provide an improved method for centering a magnetic core relative to a pole-piece assembly.

It is another object of my invention to provide a method for associating a pole-piece assembly and a bearing bracket for a'DArsonval instrument by forming coacting annular male and female registering surfaces thereon.

It is a further object of my invention to provide a method of constructing a DArsonval instrument having a pole-piece assembly, bearing brackets, a magnetic core, and a coil assembly registered by surfaces of revolution concentric about a common axis.

Other objects of my invention will be apparent from the following description when taken in connection with the accompanying drawings, in

which: Y

Figure 1 is a view in front elevation of a measuring instrument embodying my invention;

Fig. 2 is a view in side elevation of the measuring instrument illustrated in Fig. 1;

Fig. 3 is an exploded view in perspective, with parts broken away, ofthe measuring instrument illustrated in Fig. 1;

Fig. 4 is a view in front elevation of a polepiece assembly suitable for the instrument of Fig. 1;

Fig. 5 is a view in. top plan assembly illustrated inFig. 4;

Fig. 6 is a view in front elevation of a magnetic core suitable for insertion in the pole-piece assembly of Figs. 4 and 5; and

Fig. '7 is a view in bottom plan of the magnetic core illustrated in Fig. 6.

Referring to the drawings, Figure 1 shows a DArsonval measuring instrument having a U- shaped permanent magnet l positioned with its of the pole -piece While the spacer is temporarily attached to the legs abutting a pole-piece assembly 2. The magnet and pole-piece assembly may be carried by any suitable frame 3 to which they are attached in any suitable manner, as by machine screws 4. The pole-piece assembly 2 carries two bearing brackets a and 5b which may be attached thereto by machine screws 6, or in any other suitable manner. These bearing brackets carry respectively bearing screws Ia and 12) which may be threaded in tapped openings provided in the bearing brackets in order to permit adjustments thereof. Suitable lock nuts 8 may be provided for locking the bearing screws in any position to which they are adjusted.

The relationship of the various parts may be seen more clearly by reference to Fig. 3. As therein illustrated, the pole-pieceassembly 2 is provided with a central opening 9 in which a magnetic core I 9 is positioned. The magnetic core It has a diameter smaller than the opening 9 in order to provide an annular gap extending substantially around the magnetic core.

A coil I I, provided with pivots I20. and I22), is mounted in the bearings Ia and lb for rotation in the air gap between the magnetic core ii] and the pole-piece assembly 2. The coil may be of any conventional construction, but preferably it is of the type disclosed in my Patent No. 2,130,852. In this construction, the coil II is wound on a semicylindrical electroconductive form i3 (shown in dotted lines) which increases the damping of the coil assembly and increases the strength of the form on which the coil I I is wound. The coil II may be provided in a customary manner with springs I4 for controlling movements thereof, and with flexible conductors I5 for establishing contact with the terminals of the coil.

By reference to Fig. 3, it will be noted that dimensional variations in the assembly of various parts will affect the operation of the measuring instrument. For example, it is desirable to employ as small an air gap'as possible. If the magnetic core I!) is not concentrically mounted in the pole-piece assembly 2, a large air gap must be provided to accommodate the resulting dissymmetry. Moreover, if the bearing screws Ia and lb are not supported in alignment with the axis of the opening in the core 2, the coil II will not be supported symmetrically in the annular gap between the magnetic core I9 and the pole-piece assembly 2. In addition, any variation between parts of the pole-piece assembly 2 will produce corresponding variations in the response of the instrument. In order to eliminate these sources of error or variation, I have provided an improved construction which will be described in detail below.

Variations in the dimensions and positions of the parts of the pole-piece assembly 2 are prevented by constructing the pole-piece assembly from a single homogeneous block of magnetic material, as illustrated clearly in Figs. 4 and 5. The opening 9 is machined accurately in the polepiece assembly, and is illustrated as a cylindrical opening for reception of the magnetic core Ill and the coil II. In addition to the main opening 9, a number of additional openings I6 are drilled in the pole-piece assembly 2 on axes intercepting the axis of the cylindrical opening 9 at right angles. Although any desired number of openings I6 may be provided, I have illustrated three openings on each side of the pole-piece assembly. The purpose of these openings is to provide two distinct magnetic poles IT and I8 when the permanent magnet I is placed against the polepiece assembly. By removing most of the matcrial between these two poles, most of the flux passing therebetween is forced to traverse the magnetic core Ill. Only enough material is left by the openings It to provide bridges I9 having sufficient mechanical strength to prevent distortion of the pole-piece assembly 2.

The separation of the poles I1 and I8 may be enhanced by removing portions of the material between the openings I6 in order to separate ado quately the protuberances 29. The bridges It have cross-sections so small than an insignificant amount of flux can pass therethrough.

The pole-piece assembly may be provided with holes 2i for reception of machine screws 22 (see Fig. 1) which are threaded into tapped openings provided in the magnet I for securing the polepiece assembly 2 to the magnet.

Because of the unitary construction of the po3epiece assembly 2, the poles I! and I8 cannot become displaced under the influence of vibration or temperature changes. Material employed'ior the pole-piece assembly may be any conventional magnetic material, such as cold rolled steel.

Registration of the bearing brackets 5a and 5b is provided by recesses 23a and 23b formed in the pole-piece assembly 2. These recesses may be of any desired configuration, but as illustrated they are of annular shape concentric with the opening 9 and providing flat annular seats 24a and 2% and cylindrical edges 25a and 25b. The bearing brackets 5a and 5b are provided with mating protuberances 25a and 26b for these recesses. The protuberances have fiat exposed faces for en gaging the fiat faces 24a and 24b of the recesses, and have cylindrical walls for engaging the cylindrical walls 25:]. and 25b of the recesses. Consequently, each protuberance and its ccact- 'ing recess form male and female surfaces of revolution for positioning accurately the related bracket on the pole-piece assembly 2.

The brackets are provided with tapped openings 2'Ia and 212) which are formed concentric with the protuberances 26a and 2612, respectively, for reception of the bearing screws. Preferably, the openings 21a, 27b and the protuberances are formed during the same machining operation in order to assure concentricity. These bearing screws Ia and lb may be provided with bearing openings 28 which may be jeweled for reception of the bearings pivots I20; and i217.

In order to locate the magnetic core I9 accurately within the opening 9 of the pole-piece assembly 2, the magnetic core I0 first is machined accurately to size. After completion of this machining operation, a spacer 29 is positioned in a channel 30 provided in the magnetic core. To assist in locating the parts and maintaining them in position during handling, one or more pins 3| may be attached to the magnetic core and may be received in openings 32 drilled in the spacer 29. The spacer is temporarily attached to the magnetic core by machine screws 33 having heads which are received in recesses 34 provided in the spacer 2 9.

With the spacer assembled in this manner on the magnetic core, the outer surface 35 of the spacer is machined to the same radius as that of the opening 9 in order that the outer surface of the spacer may fit intimately against the portion of the wall of the opening. When the magnetic core I9 is to be mounted in the pole-piece assembly, the temporary attaching screws 33 are removed and permanent attaching screws 36 (see Fig. 1) are provided for securing the magnetic core to the pole-piece assembly. The screws 33 and 36 pass freely through the spacer 29, and are threaded in tapped openings 31 in the core 10. These tapped openings are so positioned that the screws 36 may be passed through one or more of the openings 16 in order to secure the magnetic core to the pole-piece assembly. As illustrated, two tapped openings 31 are provided which are spaced apart by a distance equal to the spacing between the outer openings IS on one face of the pole-piece assembly. The screws 36 are provided with collars 38, which are snugly received in the openings l6, and with heads 39, which are larger than the openings. Preferably, the spacer 29 and the screws 36 are of non-magnetic material, such as brass, in order to avoid magnetically bridging the poles l1 and I8 of the pole-piece assembly. Similarly, the brackets 5a and 5b may be constructed of a non-magnetic material, such as brass, for the same reason.

If desired, an adjustment for the magnetic field produced in the annular gap between the magnetic core 9 and the pole-piec assembly 2 may be provided. For example, in Fig. 1, a strip 40 of magnetic material is positioned over one of the openings IS. The strip 40 has an elongated slot 4| through which a machine screw 42 passes for attaching the strip to the pole-piece assembly. By loosening the screw 42 and moving the strip 40, the bridging action of the magnetic strip across the opening 15 which it tends to cover may be varied, as desired.

The measuring instrument herein illustrated may be employed for indicating purposes, recording purposes, or otherwise. In the specific example illustrated, a bracket 43 is attached to the upper pivot lZa for rotation therewith. This bracket may be employed for supporting a recording pen which is actuated in accordance with the movements of the coil II.

It is believed that the construction and assembly of an instrument embodying my invention are apparent from the foregoing description. The pole-piece assembly 2 may be produced by machining a circular opening 9 in the center thereof, and then drilling out sufiicient material by means of the openings IE to provide two welldefined pole-pieces l1 and I8. The annular recesses 23a and 23b are then machined on opposite surfaces of the pole-piece assembly. Each bearing bracket 5a or 51) is provided with a protuberance which closely fits the recess provided on the corresponding face of the pole-piece assembly in order to position the bracket with reference to the pole-piece assembly.

After the magnetic core I has been machined accurately to a cylindrical shape, the spacer 29 is temporarily attached thereto by means of the screws 33. This spacer then is machined while so mounted in order to conform intimately to the wall of the opening 9.

In order to insert the magnetic core in the pole-piece assembly 2, the screws 33 are removed and the coil l l is slipped over the magnetic core. The magnetic core, together with the coil, then is inserted in the opening 9, and screws 36 are passed through the opening H3 in order to attach the magnetic core l0 permanently to the polepiece assembly 2.

At this stage, the bearing brackets a and 5b may be attached to the pole-piece assembly 2 by means of the machine screws 6, and the bearing screws la and 1b may be adjusted to engage properly the pivots Her and I21) of the coil ll. When the pole-piece assembly 2 is attached to the magnet I by means of the screws 22, a completely operative measuring instrument is provided.

It will be noted that the bearing brackets 5a and 5b, the magnetic core l0 and the bearings la and 1b all are united to the pole-piece assembly 2 by surfaces of revolution having a common axis. Since a surface of revolution is a shape permitting of simple and accurate machining, it follows that the manufacture of the measuring instrument is greatly facilitated thereby. Moreover, this construction maintains the high accuracy with which the parts are positioned indefinitely, and the various parts are easily interchangeable.

Although I have described my invention with reference to certain specific embodiments thereof, it is obvious that numerous modifications thereof are possible. Therefore, I do not wish my invention to be restricted except as required by the appended claims when interpreted in view of the prior art.

I claim as my invention:

1. Th method for fitting a magnetic core to a pole-piece assembly for an electrical measuring instrument, which comprises positioning a fastening element in engagement with predetermined parts of a spacer element and a magnetic core for temporarily fastening the spacer element to a portion of the magnetic core, machining the spacer element while temporarily fastened to said magnetic core to proper dimensions for spacing said magnetic core from said pole-piece assembly, unfastening said spacer element from said magnetic core, and permanently attaching said spacer and said pole-piece assembly to said portion of said magnetic core by positioning a permanent attaching element in engagement with at least certain of said predetermined parts.

2. The method for fitting a magnetic core for an electrical measuring instrument to a polepiece assembly having an opening therein, which comprises machining a magnetic core for insertion in the opening of said pole-piece assembly, tapping an opening in said magnetic core, threading a machine screw through an opening in a spacer element into threaded engagement with said tapped opening for temporarily fastening said spacer element to said magnetic core, machining a surface of said spacer element to conform to a surface of the opening in said pole-piece assembly while said spacer element is temporarily attached tosaid magnetic core, removing said machine screw, placing said magnetic core and spacer element in the opening of said pole-piece assembly, and threading a machine screw through said pole-piece assembly and spacer element into threaded engagement with said tapped opening for permanently fastening said magnetic core to said pole-piece assembly.

3. The method for constructing a pole-piece assembly for an electrical measuring instrument, which comprises machining a substantially cylindrical opening in a homogeneous block of magnetic material, forming in said block along an axis substantially at right angles to, and intersecting, the axis of said cylindrical opening a plurality of opposed auxiliary openings for providing said block, When magnetized, with welldefined magnetic poles, positioning a fastening element in engagement with predetermined parts of a spacer element and a substantially cylindrical magnetic core having a diameter less than that of said cylindrical opening for temporarily fastening the spacer element to the magnetic core, ma-

chining said spacer element while temporarily fastened to said magnetic core to a surface proportioned to fit intimately the surface of said cylindrical opening when said magnetic core and spacer subsequently are placed therein, unfastening said temporary fastening of said spacer element to said magnetic core by said fastening element, inserting said magnetic core and said spacer element in said cylindrical opening, and passing permanent attaching means through one of said auxiliary openings into the position previously occupied by said fastening element relative to said magnetic core for permanently attaching said magnetic core and said spacer element to said block of magnetic material.

4. The method of constructing an electrical measuring instrument which comprises boring a cylindrical opening in a homogeneous block of magnetic material, perforating said block of magnetic material on opposite sides of the axis of said opening to provide opposed perforations materially decreasing the cross-section of the block of magnetic material to define opposed magnetic poles when said block of magnetic material is magnetizedfinishing a cylindrical magnetic core to a diameter providing an annular air gap of predetermined dimensions between said magnetic core and said block of magnetic material when said magnetic core is disposed in said cylindrical opening, applying a temporary fastening element for attaching to the surface of said magnetic core a spacer element extending axially of said magnetic core, said spacer element projecting from said magnetic core by a distance greater than the thickness of said air gap, finishing said attached spacer element to a cylindrical surface concentric about the axis of said magnetic core and having a radius equal to the radius of said cylindrical opening, removing said fastening element, positioning said magnetic core and said spacer element in said cylindrical opening with said spacer element abutting the wall of said cylindrical opening adjacent one of said opposed perforations, and passing attaching means through one of said perforations into space previously occupied by said fastening element relative to said magnetic core and said spacer for attaching said magnetic core and spacer element to said block of magnetic material.

5. The method of constructing an electrical measuring instrument which comprises boring a cylindrical opening in a homogeneous block of magnetic material, perforating said block of magnetic material on opposite sides of the axis of said opening to provide opposed perforations materially decreasing the cross-section of the block of magnetic material to define opposed magnetic poles when said block of magnetic material is magnetized, boring a bearing opening in each of a pair of brackets for receiving a bearing, forming cooperating male and female surfaces of revolution on a pair of faces of said block of material and each of said brackets concentric respectively with the axis of said cylindrical opening and the axis of said bearing opening for accurately positioning said bearing openings on opposite sides of said cylindrical opening, finishing .a cylindrical magnetic core to a diameter providing an annular air gap of predetermined dimension between said magnetic core and said block of magnetic material when said magneticvcore is disposed in said cylindrical opening, applying a temporary fastening element for attaching to the surface of said magnetic core a spacer element extending axially of said 'magnetic core,-said spacer element projecting from said magnetic core by a distance greater than the thickness of said air gap, finishing said attached spacer element to acylindrical surface concentric about the axis of said magnetic core and having a radius equal to the radius of said cylindrical opening, removing said fastening element, positioning a coil having bearing members for coaction with said bearing openings around said magnetic core, positioning said coil, magnetic core and said spacer element in said cylindrical opening with said spacer element abutting the wall of said cylindrical opening adjacent one of said opposed perforations, and passing attaching means through one of said perforations into space previously occupied by said fastening element relative to said magnetic core and said spacer for attaching said magnetic core and spacer element to said block of magnetic material and centering said bearing brackets by said cooperating surfaces of revolution to position said bearing members and coil accurately relative to said cylindrical opening.

LAWRENCE J. LUNAS. 

